Variable conductance trough waveguide antennas



w. Ro'rMAN 2,957,173

VARIABLE coNDucTANcE TRouGHmvEGUIDE ANTENNAS Oct. 18, 1960 2Sheets-Sheet 1 Filed March 20, 1957 1N VENTOR. A/AL fav Rar/VAN -q @ffMm @imm Oct. 18, 1960 w. ROTMAN 2,957,173

VARIABLE CONDUCTANCE TROUGH WAVEGUIDE ANTENNAS Filed March 20, 1957 2Sheets-Sheet 2 Patented Oct; 18; 1.960.`

VARIABLE coNnUcrANcE rRoUGH WAVE- GUmE ANrENNAs s Walter Rotman,Brighton, Mass. Filed Mar. zo, 1957, ser. No. 647,453

4 claims. (Cl. 343-772) (Granted under Title 3'5, U.S. VCode (1952),sec. 266) The invention described herein may be manufactured and used byor for the United States Government for governmental purposes withoutpayment to me of any royalty thereon.

This invention relates generally to antennas and more particularly toapparatus for the transmission of highfrequency electrical energy. Theinvention is characterized by the utilization of a special waveguidestructure as a means for radiating energy to the surrounding space andinvolves the use of elements within the waveguide structure forcontrolling the conductance and radiation intensity or attenuation ratein the waveguide. Control of these parameters enables the obtaining ofvarious desired controlled radiation characteristics which include alobing pattern. l A

A trough waveguide of either rectangular channel shape, U-shape, V-shapeor variations thereof having a symmetrically disposed n thereincomprises the transmission line which is modiiied to produce thecontrolled radiation. The basic concept on which this invention reliesis that anti-symmetrical obstacles in a trough waveguide couple energyfrom the bound symmetrical trough guide mode, a transverse electric (TE)mode, into energy in a transverse electromagnetic (TEM) eld whichradiates into free space from the open side of the waveguide.Symmetrical obstacles, on the other hand, react as tuning elements. Inaccordance with above principles, a variety of radiating devices may beconstructed.

The trough waveguides, together with their radiating elements, haveutility alone or as primary line source arrays for use with secondaryreflectors, horns, or lenses in radar and communication `antennasystems. In this respect, they are superior to slots or dipoleequivalent arrays on rectangular or circular waveguides.

Because of the superior electrical and mechanical properties of theantennas of this invention, many new advantages are realized. l

It is contemplated that the subject invention will open new vistas inthe iields of radio astronomy, radar, and communications.

It is an object of this invention to produce an antenna comprising atrough waveguide with means for controlling radiation therefrom. n.

It is another object of the invention to produce an antenna composed ofdiscreet resonant radiating elements..

Still another object of the invention involves the production of novelradiating, non-susceptive elements whose conductances are variable.

It is a further object of the invention to produce an antenna which iseasily and economically produced by conventional, commercialmanufacturing techniques.

It is a still further object of this invention to produce a novelantenna for broadband radiation.

Another object of the invention is to produce a novel resonant radiatingelement for use in a trough waveguide.

Still another object of the invention involves the production of a noveltraveling wave antenna array of resonant radiating elements.

y I Y, 2 y j. K A further object of the invention involves theproduction of a novel antenna suitable for producing a sequential lobingradiation pattern. Y A still further object of the invention involvesthe production of an antenna array where a phase reversal is achievedbetween successive radiating elements.

These and other advantages, features and objects ofthe invention willbecome more apparent from the following description taken in connectionwith the villustrative embodiments in the accompanying drawings,wherein:

'Figure l is a cross-sectionalview of a trough waveguide with arepresentation of the electric eld -of the transverse electric (TE) modetherein; y

Figure 2 is a cross-section of a trough waveguide with a representationof the electric componentsf ofthe transverse electromagnetic (TEM)-eld;p 1

Figure'3 is a cross-section of a trough waveguide with a TE mode to TEM`field coupler; d

Figure 4 s a cross-sectional View of a trough Waveguide with a tuningelement for ,the TE mode;

Figure 5 is a pictorial view of the tri-rod coupling element; Y j f lFigure 6 is a pictorial view of a rotatable. tri-rod coupler in a troughwaveguide; and n f Figure 7 is a schematic representationof Va part of atrough waveguide with variable coupling elements for lobe switching. v jl, Y Thev invention utilizes a symmetrical non-radiating troughwaveguide which acts as a transmission line. vThis type of waveguide in`its mechanical construction is left open on one side; however, it stillacts as a transmission line. Its characteristics, frequency-wise followthose-ot a waveguide while it retains simplicity of a strip trans-.mission line. The general conguration of the waveguide has ltwo sidewalls and a bottom, thus forming a trough, and a substantially centrallydisposed lin member of less height than the ,side walls.l The embodimentof Figure 1 is a channel or rectangular form of trough waveguide butvarious alternative formsv can be achieved by deformation of thesidewalls, for example, the troughV v may be of U or V shapeas long assymmetry about the centraln is maintained in order to avoid spuriousmodes. Like numerals will designate like parts ofthe wave-l guidethroughout the specification. t p. Figure 1, for illustration purposes,shows a rectangular Y trough waveguide having side Wallsy 20 and 21, abottom wall 22 and a substantially centrally disposed iin member 23.lWhen the side walls 20 and 21 are lessV than one-half wavelength apart,a TE (transverse electric) mode may be propagated along the axis of theguide. This mode is bound Vto the center lin and has an electric iieldwith a general coniguration as shown in FigureV 1.,A The intensity ofthe eld lines of the electric vector increases from the bottom 22 of thewaveguide to the top of the central vane or Viin 23. The transversecurrents on the sides of n 23 vary from a minimum at the free edge to amaximum at its base. v

The electric components of the TEM (transverse electroma'gnetic) fieldis depicted in Figure 2 and-can bel propagated wherever side walls existasis shown by -the verted into a TEM field which is not bound to the,centery fin. The'TEM eld is capable of being propagated in`v alldirections at right angles to the TE vectors. which allows a release ofenergyfrom the open side of the wave-- guide.`

Although the trough waveguide is shown as ,a` unitary.A

structure of highly conductive material,y each of portions' designated'as20, 21, 22`and'23 couldbemade of separate ieces of stock of anymaterial secured together by con- 'entional means as long as theinterior of the trough is lated or otherwise lined with a highlyconductive maerial.

The* characteristics of a trough waveguide ar-e such hat the cut-offwavelength depends upon the electrical leight of the center n 23, e.g.,the cut-olf wavelength s approximately that at which the center tin is aquar er wavelength. The side wall (20, 21) height abovev he center fin23 and the spacing between side walls act4 o prevent unwanted,uncontrolledy radiation. Less than ralf wavelength spacing between theside walls allows :'or operation of the line over a range offrequencies'. lhe spacing of the side walls, at any rate, should not:Xceed a half wavelength of the highest frequency in he range. The TEmode is critically dependent upon he dimensions ofthe center iin whilethe TEM or radiatng mode is independent of the n 23.

Figure 3 represents a basic principle or concept upon which thisinvention is based. An anti-symmetrical ob- ;tacle, such as a horizontalrod 24, has the ability in a rough waveguide to convert some of theenergy from he bound TE mode into the TEM field which is then radiatedinto free space. Utilization of this principle allows for the productionof controlled radiation along :he length of the waveguide. The rod 24,perpendicular :o center iin 23, creates an asymmetry such that when rod24 is excited, it reradiates the incident energy in both the TEM fiel-dand the TE mode thereby causing a portion of the incident energy fromthe TE mode to be converted into the TEM eld while another portion isreflected back in the TE mode towards the source. The shunt admittanceof the rod as measured in the trough guide appears to have twocomponents: a reactive suscept-ance determined by the amount ofreflected energy and a conductance governed by the amount of energycoupled between the TE mode and the TEM field and subsequently radiated.The rod 24 will appear as a resonant element if the capacitivesusceptance is cancelled by an additional reilection, equal in magnitudebut opposite in phase. Changing the length of the rod increases both thesusceptive and conductive component.

Symmetrical obstacles do not couple the TE mode and TEM field;therefore, they may be used as tuning elements. The vertical post 25placed above fin 23 (Figure 4) is greater than a quarter wavelength andby virtue of its symmetry does not couple energy, but causes areflection which appears as an inductive shunt susceptance.

The tri-rod coupler 26 (Figure 5) may be applied to a waveguide asillustrated in Figure 6. The coupling of tri-rod 26 may be variedcontinuously from zero to some maximum value while its susceptanceremains substantially zero. Rotation ot the coupler allows for theproduction of various` desired, radiation amplitudes and formodification of the radiation pattern.

Coupler 26 comprises an inductive tuning post 27, symmetrical capacitivetuning rods 28 and 29, and an asymmetrical coupling rod 30. By combiningelements 24 and 25 of Figures 3 and 4 the structure of Figure 5 resultswithout elements 28 and 29. Rotation of the two element structurescomprising elements 27 and 30 mounted on the fin 23 of a waveguide willbe considered. Capacitive asymme-tric rod 30, when at a position atright angles to `the lin 23, may be resonated by adjusting the length ofvertical post 27. When element 30 lies on top of and in line with the.fin 23, a lack of asymmetry occurs yand the conductance drops to zerowith only a residual susceptance, due to the vertical post 27 remain-Identical elements 28 and 29, at right angles to the inductive post 27and on either side thereof, compensate for the residual susceptance,effectively reducing it to substantially zero.

Asshown in Figure;6,.thetri.1:o,d coupler 26. is mounted on a shaft 31housed by a tubular por-tion 32 forming a part of the center iin 23 4in*the trough waveguide; By passing the shaft 31 through` center fin 23,the rotation of the shaft does not disturb the field within the guide.Rotation of shaft 31 produces a rotation of coupler 26, which, becauseof its symmetry, remains approximately in resonance in all of itsorientations and the conductance varies smoothly and cyclically fromzero -to a maximum value.

By placing a number of tri-rod couplers 26, properly oriented, along thetin, the variations in phase and conductance produced by the diierentorientations of the A coupler may be utilized to produce variousradiation characteristics. For example, a broadside radiating antennamay be constructed by utilizing the 180 phase reversal obtained byhaving rod 30 project on opposite sides of lin 23 at half wavelengthintervals.

A holding means may be provided for the shaft 32 (e.g., a set screw inthe projecting portion of tubular portion 32 below wall 22) wheremovement of the cou-` pler will not be affected at too frequentintervals. For relatively long periods of fixed positions of -thecoupler, post 27 of the tri-rod coupler could house a 'screw meanstherein which would engage fin 23 thus securing the. coupler to the linwithout the need of a shaft or housing.`

By utilizing the rotating means displayed in Figure 6, the tri-rodcoupler may be used for several diierent types of sequential lobingantenna systems.

Referring to the schematic representation of Figure 7" which shows onlythe center fin of a trough waveguide, couplers 26 `are placed anintegral number m of half" wavelengths apart with a shorting termination33 placedl an odd number n of quarter wavelengths on the n 231 beyondthe second coupler. A signal source 34 feeds the waveguide. The shaftsof the two couplers are gearedJ together as shown by gears 35, 3o and 37and oriented so that one coupler is mechanically out of phase with theother. Thus, when attached -to a driving means; 38 and the tirst coupleris at its maximum conductance, the second element is completelydecoupled, and vicel versa. Since the elements are spaced a halfWavelength apart, the input admittance to the trough waveguide is,A thesum of the admittances of the two couplers, It can be seen that as thecouplers are rotated in synchronism, the input conductance remainsapproximately constant while the susceptance is always signioant. Thus,this" antenna system always presents a good match to the transmitter.However, the apparent point off radiation moves from the first couplerto the second and back' again as the elements yare rotated, completingtwo cycles for one rotation of the couplers. If this device is used asthe primary feed for sa microwave lens or reflector, sequential lobingof the radiation pattern may be obtained.

Although the invention has been described with reference to particularembodiments, it will be understood to those skilled in the art that theinvention is capableof a variety of alternative embodiments within thespirit and scope of the appended claims.

I claim:

1. A waveguide adapted for the controlled interchange of energy betweenitself and surrounding space comprising a transmission line of generallytrough-shaped cross section which is substantially constant throughoutits length, a center fm symmetrically disposed within said' generallytrough-shaped structure and a tri-rod coupling element mounted on saidcenter tin, said tri-rod coupler comprising a horizontal radiating orcoupling element, a tuning element` at right angles to said radiatingelement and a pair of capacitive tuning elements each of which ismutually perpendicular to both said radiating element' and said tuningelement.

2. A waveguide as defined in claim -1 wherein a number of said tri-rodcouplers are oriented on said' center n.

3. A waveguide as defined in claim 2 including means for adjusting theorientation of said tri-rod couplers.

4. A sequential lobing antenna comprising a generally trough-shapedwaveguide having a centrally disposed n member extending longitudinallythereof, at least one pair of tri-rod couplers mounted on said n anintegral number of half wavelengths apart, and means -for rotating saidtri-rod couplers simultaneously in a 90 mechanically out of phaserelation, said tri-rod coupler comprising a horizontal radiating orcoupling element, a vertical tuning element attached to one end of saidradiating element and a pair of capacitive tuning elements each of whichis perpendicular to said radiating element and said vertical tuningelement.

References Cited in the le of this patent UNITED STATES VPATENTS OTHERREFERENCES Pub. I. Some New Microwave Antenna Designs Based on theTrough Wave Guide, Rotman and Karas, 1956 IRE Convention Record, vol. 4,Part l, copyright l1956,

5 pp. 23o-235.

Pub. II. The Bell System Technical Journal, vol. 34, No. 1 January 1955,pp. 71, 72.

